Well everyone go out and see how many MB dealers will sell a RB for the proper list price of 1750. A 4 grand estimate is more likely. I don't know how it gets that way but its is more realistic.
As to wholesale, MB rebuilt prices are special catagory. A good wholesale relationship will have one buying at a percent over MB net (not at the middle column MB wholesale). MB whole sale is usually at about 15 to 20% off list. I buy most of my next day orders at net plus 20%. This is 28% off list. I pay net plus twenty for rebuilt also, but here is an example: 103 fuel distributor MB list 676. My cost 606 (plus or minus a dollar or two at net plus 20%). This is about 10% off list and is the way it is with a tranny. It is the main reason our rebuilds with install and fluid cost 1600. If we were at 2000 we would only be a couple hundred less than the factory rebuild sold by a dealer at MB suggested pricing and warrantee based labor (all MB trannies pay more than 5 hours warrantee - all other labor guides give at least another hour - we charge 6 hours R&R as part of our total).
As to the repair in question, do you believe the tech to be dishonest? The senario he describes is very concievable. The questions he asks are due to what he sees once apart; hardly something he could know ahead of time. I won't do a front pump reseal on a tranny with over 150k so he is probably justifiable in doing it with your mileage. Unfortunately mileage is just one component of age and often low mileage is not a benefit. Many seal type problems originate from no use. Rubber takes a "set" and/or dryes up. Corrosion takes place in bushings that leads to premature failure Don't expect him to be a metallurgist, why is not necessarily his job.
Without seeing the problem I can't really advise but the question of rebuilding is entirely different once this far involved. I would say that half of the labor has already been invested and unless you don't trust the man, I would go with his suggestions. It is most likely that what he is trying to prevent is for your benefit not his. In other words, he is not trying to get you to pay more to protect his warrantee. His seal repair will make it through the clutch failure that probably wasn't (or isn't) immenent. But if it fails in three years the seal repair will become a waste.
Soopsmart has some interesting figures that are basically correct, but the shop in question can not sell you a MB rebuilt at 1750 and keep its door open (especially after spending time disassembling your tranny - which will have to be reassembled tobe acceptable as a core) and neither do any dealers. For DIY it is probably possible to get a tranny for less than the 1750 from a dealer not having to install, but if my dealer doesn't do it in a rural setting I doubt anyone does it with installation.
Bosch Master, ASE Master, L1
33 years MB technician